Austria is great for swimming out in the open, the lakes are clean with a fantastic panorama around as a setting. The Salzburg tourist office gives regular updates of the water temperature and the facilities in and around the lake.
For example at the time of writing the temperature of the lake at Zell am See was given at 20 degrees centigrade, and a water quality of "A" which is a drinking water standard. The site tells you also what facilities beaches, boats etc. are available and open.
For those who are tempted by wilder waters it is possible to go Canyoning, walking up the river the wrong way in a wet suit, and then swimming / walking / floating back down. Sometimes this is done with the benefit of a rubber ring or other flotation ring. For further information on canyoning, rafting and other advanture sports click on the Adventure Sports link. There is also the Rafting Centre at Taxenbach, they not only do rafting, but they will organize "Pirate Rafting" aimed at younger adventuresrs, along with mountain biking, parascending. It is possible to do a session of cycling, canyoning and wild water rafting in one day.
If you are looking for somewhere to stay for your group please consider staying at Austrian holiday apartment in Kaprun, close to Zell am See, with lots to do in and around the area.